I'm wondering why don't you just use HarfBuzz? It's MIT licensed, so it's bright from a legal side. And it's even used in many products, Chromium as an example. I purchased a license for Affinity Designer and was thinking of leaving Photoshop to Affinity Photo but it seems like I made a wrong decision. I even suggested Affinity apps for some colleagues considering them as well-made apps as I thought they were in-place replacements for adobe's apps, but I was wrong.
At least share your roadmap with us so we can know where exactly this feature is.